Solid carbon products comprising carbon nanotubes and methods of forming same

ABSTRACT

Methods of forming solid carbon products include disposing a plurality of nanotubes in a press, and applying heat to the plurality of carbon nanotubes to form the solid carbon product. Further processing may include sintering the solid carbon product to form a plurality of covalently bonded carbon nanotubes. The solid carbon product includes a plurality of voids between the carbon nanotubes having a median minimum dimension of less than about 100 nm. Some methods include compressing a material comprising carbon nanotubes, heating the compressed material in a non-reactive environment to form covalent bonds between adjacent carbon nanotubes to form a sintered solid carbon product, and cooling the sintered solid carbon product to a temperature at which carbon of the carbon nanotubes do not oxidize prior to removing the resulting solid carbon product for further processing, shipping, or use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is divisional of U.S. patent application Ser. No. 14/414,232, filed Jan. 12, 2015, which is a national phase entry under 35 U.S.C. §371 of International Patent Application PCT/US2013/049719, filed Jul. 9, 2013, designating the United States of America and published in English as International Patent Publication WO 2014/011631 A1 on Jan. 16, 2014, which claims the benefit under Article 8 of the Patent Cooperation Treaty to U.S. Provisional Patent Application Ser. No. 61/671,022, filed Jul. 12, 2012, for “Solid Carbon Products Comprising Carbon Nanotubes and Methods of Forming Same,” the disclosure of each of which is hereby incorporated herein in its entirety by this reference.

TECHNICAL FIELD

Embodiments of the present disclosure relate to methods and systems for forming solid carbon products from carbon nanotubes including mixtures of various types of carbon nanotubes and mixtures of carbon nanotubes with other substances.

BACKGROUND

The following documents, each published in the name of Dallas B. Noyes, disclose background information hereto, and each is hereby incorporated herein in its entirety by this reference:

-   -   1. U.S. Patent Publication No. 2012/0034150 A1, published Feb.         9, 2012;     -   2. International Application No. PCT/US2013/000071, filed Mar.         15, 2013;     -   3. International Application No. PCT/US2013/000072, filed Mar.         15, 2013;     -   4. International Application No. PCT/US2013/000073, filed Mar.         15, 2013;     -   5. International Application No. PCT/US2013/000075, filed Mar.         15, 2013;     -   6. International Application No. PCT/US2013/000076, filed Mar.         15, 2013;     -   7. International Application No. PCT/US2013/000077, filed Mar.         15, 2013;     -   8. International Application No. PCT/US2013/000078, filed Mar.         15, 2013,     -   9. International Application No. PCT/US2013/000079, filed Mar.         15, 2013; and     -   10. International Application No. PCT/US2013/000081, filed Mar.         15, 2013.

Conventional methods of using CNTs (“carbon nanotubes”) in engineering materials generally rely on embedding the CNTs in a matrix material. CNTs are currently processed in a wide variety of composite structures using metals, plastics, thermoset resins, epoxies, and other substances as the matrix to hold the CNTs together, thus creating solid objects. The CNTs act as reinforcing material to improve properties of the materials. Typical objectives of using carbon nanotubes in a matrix are to increase the strength, decrease weight, or to increase electrical and thermal conductivity of the composite.

Methods to make materials composed primarily of carbon nanotubes include spinning the carbon nanotubes into fibers and making “buckyrock.” U.S. Pat. No. 6,899,945, issued May 31, 2005, and entitled “Entangled single-wall carbon nanotube solid material and methods for making same” discloses a method for making buckyrock. Buckyrock is a three-dimensional, solid block material including an entangled network of single-wall CNTs. Buckyrock is mechanically strong, tough, and impact resistant with a bulk density of about 0.72 g/cm³ (see Example 3 of U.S. Pat. No. 6,899,945). The single-wall CNTs in a buckyrock form are present in a random network. The random network of the CNTs appears to be held in place by Van der Waals forces between the CNTs and by physical entanglement or interference of the CNTs. One type of buckyrock is made by forming a slurry of CNTs in water, slowly removing water from the slurry to create a paste, and allowing the paste to dry very slowly, such that the CNT network of the paste is preserved during solvent evaporation. Buckyrock can be used in various applications requiring lightweight material with mechanical strength, toughness, and impact resistance, such as ballistic protection systems.

Though conventional materials including CNTs have interesting and useful properties, the individual CNTs comprising these materials have significantly different properties. It would therefore beneficial to produce materials having properties more comparable to the properties of individual CNTs.

BRIEF SUMMARY

Methods of forming solid carbon products include pressure compaction methods such as extruding, die pressing, roller pressing, injection molding etc. to form solid shapes comprising a plurality of carbon nanotubes. The carbon nanotubes may optionally be mixed with other substances. Such solid shapes may be further processed by heating in an inert atmosphere to temperatures sufficient to sinter at least some of the CNTs so that covalent bonds form between adjacent CNTs. The methods may include forming a plurality of nanotubes, disposing the plurality of nanotubes in a press, and applying heat and pressure to the plurality of carbon nanotubes to form the solid carbon product. When sintered, the resulting material is a novel composition of matter having two or more CNTs with covalent bonding between them.

The solid carbon products, whether sintered or not, include interlocked CNTs that define a plurality of voids throughout the material. The dimension of the interstitial voids may be controlled by a variety of methods including controlling the characteristic diameter of the CNTs comprising the solid carbon products, the inclusion of other materials that may create voids when removed from the solid carbon products, and the pressure and temperatures at which the solid carbon products are formed.

Sintered solid carbon products include a plurality of covalently bonded carbon nanotubes. Some methods include compressing a material comprising carbon nanotubes, heating the compressed material in a non-reactive environment to form chemical bonds between adjacent carbon nanotubes and form a bonded carbon nanotube structure, and cooling the bonded carbon nanotube structure to a temperature at which carbon of the carbon nanotubes does not react with oxygen.

Other methods include first forming a solid carbon product by compressing a material comprising carbon nanotubes and subsequently placing the resulting solid carbon product into sintering conditions. The sintering conditions may include an inert environment, such as a vacuum or inert atmosphere (e.g., argon or helium). The solid carbon product is heated to a desired temperature for a period of time to induce covalent bonding between adjacent CNTs, after which the object is cooled below the oxidation temperature of carbon in air. The product may then be removed from the sintering conditions.

Such methods may include any of a variety of standard industrial processing methods such as extrusion, die pressing, injection molding, isostatic pressing, and roll pressing. The sintering of the solid carbon products can be performed in a variety of apparatus such as are commonly used in sintered powder metallurgy and sintered ceramic processing. The sintering of the solid carbon products may include any of a variety of means including induction heating, plasma arc discharge, high temperature autoclaves and annealing furnaces, and other related devices and methods as are known in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 are simplified illustrations of carbon nanotubes;

FIGS. 5 through 9 are simplified cross-sectional views of presses for forming solid carbon products;

FIGS. 10 and 11 are simplified illustrations depicting the structures of linked carbon nanotubes; and

FIG. 12 is a graph showing bulk densities of solid carbon products formed by compaction and sintering.

DETAILED DESCRIPTION

This disclosure includes methods of forming solid carbon products by applying pressure to carbon nanotubes, and to methods for applying heat to the solid products formed by such processes. Solid carbon products may be useful in various applications, such as filters, reactors, electrical components (e.g., electrodes, wires, batteries), structures (e.g., beams, frames, pipes), fasteners, molded parts (e.g., gears, bushings, pistons, turbines, turbine blades, engine blocks), etc. Such solid carbon products may exhibit enhanced properties (e.g., strength, electrical or thermal conductivity, specific surface area, porosity, etc.) with respect to conventional materials. This disclosure includes a new class of materials that contain a plurality of CNTs formed into solid shapes under pressure. When such solid shapes are sintered, covalent bonds form between at least some of the CNTs, forming solid shapes. This material has numerous useful properties.

As used herein, the term “sintering” means and includes annealing or pyrolyzing CNTs at temperatures and pressures sufficient to induce carbon-carbon covalent bonding between at least some of the adjacent CNTs between at least some of their contact points.

As used herein, the term “catalyst residual” means and includes any non-carbon elements associated with the CNTs. Such non-carbon elements may include a nanoparticle of a metal catalyst in the growth tip of the CNTs, and metal atoms or groups of atoms randomly or otherwise distributed throughout and on the surfaces of the CNTs.

As used herein, the term “green” means and includes any solid carbon product that has not been sintered.

CNTs may be created through any method known to the art, including arc discharge, laser ablation, hydrocarbon pyrolysis, the Boudouard reaction, the Bosch reaction and related carbon oxide reduction reactions, or wet chemistry methods (e.g., the Diels-Alder reaction). The methods described herein are applicable to carbon nanotubes regardless of the method of manufacture or synthesis.

CNTs may occur as single-wall and multi-wall carbon nanotubes of various diameters ranging from a few nanometers to 100 nanometers in diameter or more. CNTs may have a wide variety of lengths and morphologies, and may occur as substantially parallel “forests”, randomly tangled masses, or “pillows” of structured agglomerations. CNTs may also form or be compounded to form many different mixtures of CNTs with various combinations and distribution of the above characteristics (number of walls, diameters, lengths, morphology, orientation, etc.). Various mixtures, when compounded and used to form the solid carbon products described herein, may result in products with specifically engineered properties. For example, the median void size of interstitial spaces between CNTs comprising solid carbon products typically is approximately proportional to the characteristic diameters of the CNTs used in forming the solid carbon products. The median void size influences the overall porosity and density of the solid carbon products.

Various CNT features and configurations are illustrated in FIGS. 1 through 4. FIG. 1 shows a single-wall CNT 100, in which carbon atoms 102 are linked together in the shape of a single cylinder. The carbon atoms 102 are covalently bonded into a hexagonal lattice, and thus form a CNT 100 that appears as a single graphitic layer rolled into the form of a tube. The CNT 100 may be conceptualized as a “rolled graphene sheet” lattice pattern oriented so that the carbon atoms 102 spiral at various angles with regard to the axis of the CNT 100. The angle is called the “chirality” and common named forms include armchair and zigzag, as described in Mildred S. Dresselhaus & Phaedon Avouris, Introduction to Carbon Materials Research, in Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, 1, 6 (Mildred S. Dresselhaus, Gene Dresselhaus, & Phaedon Avouris, eds., 2001), the entire contents of which are incorporated herein by this reference. Many chiralities are possible; CNTs 100 with different chiralities may exhibit different properties (e.g., CNTs 100 may have either semiconductor or metallic electrical properties).

The CNT 100 has an inside diameter related to the number of carbon atoms 102 in a circumferential cross section. The CNT 100 depicted in FIG. 1 has a zigzag pattern, as shown at the end of the CNT 100. The diameter may also affect properties of the CNT 100. Single-walled CNTs 100 can have many different diameters, such as from approximately 1.0 nm (nanometer) to 10 nm or more. A CNT 100 may have a length from about 10 nm to about 1 μm (micron), such as from about 20 nm to about 500 nm or from about 50 nm to about 100 nm. CNTs 100 typically have an aspect ratio (i.e., a ratio of the length of the CNT to the diameter of the CNT) of about 100:1 to 1000:1 or greater.

CNTs having more than one wall are called multi-wall CNTs. FIG. 2 schematically depicts a multi-wall CNT 120 having multiple graphitic layers 122, 124, 126, 128 arranged generally concentrically about a common axis. Double-walled and triple-walled carbon nanotubes are occasionally described as distinct classes; however, they may be considered as the smallest categories of multi-walled CNTs 120. Diameters of multi-wall CNTs 120 can range from approximately 3 nm to well over 100 nm. Multi-wall CNTs 120 having outside diameters of about 40 nm or more are sometimes referred to as carbon nanofibers in the art.

FIG. 3 depicts two forms of multi-wall CNTs 140, 150. In the CNT 140, one single-wall CNT 142 is disposed within a larger diameter singe-wall CNT 144, which may in turn be disposed within another even larger diameter single-wall CNT 146. This CNT 140 is similar to the CNT 120 shown in FIG. 2, but includes three single-wall CNTs 142, 144, 146 instead of four. Another form of multi-wall CNTs shown in FIG. 3 is CNT 150, which may be conceptualized as a single graphene sheet 152 rolled into tubes.

FIG. 4 schematically depicts a single-wall CNT 180 with an attached nanobud 182. The nanobud 182 has a structure similar to a spherical buckminsterfullerene (“buckyball”), and is bonded to the single-wall CNT 180 by carbon-carbon bonds. As suggested by the structure shown in FIG. 4, modifications may be made to the wall of a single-wall CNT 180 or to the outer wall of a multi-wall CNT. At the point of bonding between the nanobud 182 and the CNT 180, carbon double-bonds can break and result in “holes” in the wall of the CNT 180. These holes may affect the mechanical and electrical properties of the CNT 180. In single-wall CNTs, these holes may introduce a relative weakness when compared unmodified cylindrical CNTs. In multi-wall CNTs, the outer wall may be affected, but any inner walls likely remain intact.

Carbon nanotubes are typically formed in such a way that a nanoparticle of catalyst is embedded in the growth tip of the carbon nanotube. This catalyst may optionally be removed by mild washing (e.g., by an acid wash). Without being bound to a particular theory, it is believed that if the catalyst is left in place, catalyst atoms become mobilized during the sintering process, and may migrate to the surface or within the pores of the carbon nanotubes. This process may disperse the catalyst atoms randomly, uniformly, or otherwise throughout the solid carbon product mass and may have a significant influence on the properties of the solid carbon product. For example, catalyst material may affect electrical conductivity or the ability to catalyze other chemical reactions.

The catalyst particles may be selected to catalyze other reactions in addition to the formation of solid carbon. Catalyst particles may be any material, such as a transition metal or any compound or alloy thereof. For example, catalyst particles may include nickel, vanadium oxide, palladium, platinum, gold, ruthenium, rhodium, iridium, etc. Because the catalyst particles are attached to or otherwise associated with CNTs, each catalyst particle may be physically separated from other catalyst particles. Thus, the catalyst particles may collectively have a much higher surface area than a bulk material having the same mass of catalyst. Catalyst particles attached to CNTs may therefore be particularly beneficial for decreasing the amount of catalyst material needed to catalyze a reaction and reducing the cost of catalysts. Compressed solid carbon products used as catalysts may, in many applications, benefit from the catalytic activity of both the CNT and the metal catalyst particles embedded in the growth tip of the CNTs.

The CNTs used in the processes herein may be single-wall CNTs, multi-wall CNTs, or combinations thereof, including bi-modally sized combinations of CNTs, mixtures of single-wall and multi-wall CNTs, mixtures of various sizes of single-wall CNTs, mixtures of various sizes of multi-wall CNTs, etc. The CNTs may be in forms such as a sheet-molded compound, a pressure-molded compound, or as a pourable liquid. The CNTs may be disposed within a press any other device structured and configured to provide pressure to the material. The press may include an extrusion die, a mold, a cavity, etc.

For example, in the press 200 shown in FIG. 5, CNTs 202 may be placed in a hopper 204 configured to feed material through an extrusion die 206. The press 200 includes an extrusion barrel 208 with a screw mechanism 210 connected to a drive motor 212 to carry the CNTs 202 through the extrusion barrel 208 to the extrusion die 206. The extrusion barrel 208 may optionally include means for heating the CNTs 202 as the CNTs 202 pass through the extrusion barrel 208. The extrusion die 206 has an opening with a shape corresponding to the cross-sectional shape of a part to be formed in the press 200. Extrusion dies 206 may be interchangeable, depending on the shape of objects desired. Some possible shapes of extrusion dies 206 a, 206 b, 206 c are shown. For example, the extrusion die 206 may have an opening shaped like a circle, a regular polygon, an irregular polygon, an I-beam, etc. Extrusion dies 206 can be structured to create objects of extruded CNTs of a variety of shapes and sizes: symmetrical or asymmetrical, small to large. The CNTs 202 may optionally be mixed with another material before or within the press 200.

In some embodiments and as shown in the press 220 of FIG. 6, the CNTs 202 are placed into a hopper 224 configured to feed material to a mold 226. The press 220 includes a barrel 228 with a screw mechanism 230 connected to a drive motor 232 to carry the CNTs 202 through the barrel 228 to the mold 226. The barrel 228 may optionally include means for heating the CNTs 202 as the CNTs 202 pass through the barrel 228. The mold 226 has an opening with an interior shape corresponding to the exterior shape of a part to be formed in the press 220. Molds 226 may be interchangeable, depending on the shape of objects desired. Some possible shapes of molds 226 a and 226 b are shown. For example, the mold 226 may have a shape of a screw or a propeller. The CNTs 202 may optionally be mixed with another material before or within the press 200 to improve flowability, mold release, or other process properties. Such materials may be subsequently removed by suitable means such as etching, pyrolysis, evaporation, etc. The resulting solid carbon product may substantially free of the additional material, and may include essentially carbon and, in some embodiments, residual catalyst material.

In other embodiments and as shown in the press 240 of FIG. 7, the CNTs 202 are placed into a body 244 having an interior shape defining an exterior of a product to be formed. The CNTs 202 may be placed between two pistons 246, 248 surrounded by the body 244. The body 244 may have walls 250 defining an interior cavity and configured to allow the pistons 246, 248 to slide freely. In other embodiments, a single piston may be configured to press CNTs against a body.

In an embodiment as shown in the press 260 of FIG. 8, CNTs 202 are placed within a mold portion 262 having one or more surfaces corresponding to a shape of a product to be formed. One or more additional mold portions 264 are configured to press the CNTs 202 against the mold portion 262, when pressed by pistons 266, 268, as shown in FIG. 9. Together, the mold portions 262, 264 define the shape of the product to be formed.

Pressure is applied to form the CNTs into a cohesive “green” body. For example, the screw mechanisms 210, 230 shown in FIGS. 5 and 6 apply pressure to the CNTs 202 as the CNTs 202 pass through the presses 200, 220. Extrusion through a die 206 as shown in FIG. 5 may be continuous (theoretically producing an infinitely long product) or semi-continuous (producing many pieces). Examples of extruded material include wire, tubing, structural shapes, etc. Molding, as in the press 220 shown in FIG. 6, is the process of manufacturing by shaping pliable raw material (e.g., CNTs 202) using a rigid pattern (the mold 226). The CNTs 202 adopt the shape of the mold.

The pistons 266, 268 shown in FIGS. 8 and 9 are pressed toward the CNTs 202, forming the CNTs 202 into a green body 270. The resulting green body 270 formed may be held together by relatively weak forces, such that the green body 270 may easily be further shaped (e.g., machined, drilled, etc.), but still holds its shape when handled. The CNTs of the green body 270 may each be in physical contact with one or more other CNTs.

Heat is applied to green bodies to link the CNTs together into a more cohesive body in which at least some of the adjacent CNTs form covalent bonds between one another. For example, the CNTs may be heated at a heating rate from about 1° C./min to about 50° C./min to a temperature of at least 1500° C., 1800° C., 2100° C., 2400° C., 2500° C., 2700° C. or even to just below the sublimation temperature of carbon (approximately 3600° C.). Pressure may also be applied concurrently with, before, or after heat is applied. For example, the CNTs may be pressed at 10 to 1000 MPa, such as 30 MPa, 60 MPa, 250 MPa, 500 MPa, or 750 MPa. The green bodies may be subjected to a heated inert environment, such as helium or argon, in an annealing furnace. Sintering CNTs (i.e., subjecting them to heat in an oxygen-free environment) apparently creates covalent bonds between the CNTs at points of contact. The sintering of the CNTs typically occurs in a non-oxidizing environment, such as a vacuum or inert atmosphere so that the carbon nanotubes are not oxidized during the sintering. Sintering CNTs to induce chemical bonding at the contact surfaces may improve desirable material properties such as strength, toughness, impact resistance, electrical conductivity, or thermal conductivity in the solid structure product when compared to the green material. The CNTs may also be sintered in the presence of additional constituents such as metals or ceramics to form composite structures, lubricants to aid processing, or binders (e.g., water, ethanol, polyvinyl alcohol, coal, tar pitch etc.). Materials may be introduced as powders, shavings, liquids, etc. Suitable metals may include, for example, iron, aluminum, titanium, antimony, Babbitt metals, etc. Suitable ceramics may include materials such as oxides (e.g., alumina, beryllia, ceria, zirconia, etc.), carbides, boride, nitrides, silicides, etc. In embodiments in which materials other than CNTs are present, covalent bonding occurs between at least some of the CNTs, and the additional materials may become locked into a matrix of CNTs.

The CNTs in the sintered body have chemical bonds connecting one another. Chemical bonds, which are generally stronger than physical bonds, impart different properties on the collection of CNTs than physical bonds. That is, the sintered body may have higher strength, thermal conductivity, electrical conductivity, or other properties than the green body from which it was formed.

When single-wall CNTs are covalently bonded to adjacent single-wall CNTs, holes can form on the surface of the CNTs as some of the carbon-carbon bonds break, thus modifying the mechanical and electrical properties of each single-wall CNT. The sintered single-wall CNTs, however, may still typically exceed non-sintered single-wall CNTs in such properties as strength, toughness, impact resistance, electrical conductivity, and thermal conductivity. With multi-wall CNTs, typically only the wall of the outer tube is modified; the internal walls remain intact. Thus, using multi-walled and bi-modally sized CNTs in, for example, extrusion and molding processes, may yield solid structures with properties that, in many respects, exceed practical limitations of single-walled CNTs.

Sintering appears to cause covalent bonds to form between the walls of CNTs at their contact points. That is, any given CNT may “cross-link” with an adjacent CNT at the physical point of contact of the two CNTs. Any given CNT having undergone sintering may be covalently bound to numerous other CNTs (both single-wall CNTs and multi-wall CNTs). This increases the strength of the resulting structure because the CNTs do not slide or slip at the bonding points. Unsintered, CNTs (e.g., in buckyrock) may slide with respect to each other. Because the covalent bonding caused by sintering may occur at numerous sites in the mass of CNTs, the sintered body has significantly increased strength, toughness, impact resistance, and conductivity over convention agglomerations of CNTs.

FIG. 10 schematically depicts the cross-linked structure of two covalently bound CNTs 280, 282 produced by sintering. When sintered, CNTs 280, 282 covalently bond at their contact points 284. Each CNT may form covalent bonds with some or all of the other CNTs with which it is in contact during sintering. Due to the internal layering in a multi-wall CNT, covalent boding between the individual walls of the multi-wall CNT is likely to occur under sintering conditions. However, this covalent bonding has not yet been confirmed in testing. The heating and optional pressurization of the CNTs in a sintering process are maintained until the desired level of cross-linking has occurred. The sintered CNTs are then cooled to a temperature at which the CNTs will not spontaneously react with oxygen. Thereafter, the mixture may be exposed to air for further processing, storage, packaging, shipment, sale, etc.

In another embodiment, a CNT mixture is heated in a reactive environment (e.g., in the presence of oxygen, hydrogen, a hydrocarbon, and/or another material). In this embodiment, heat and pressure are maintained as needed until the reactants in the reactive environment have reacted with one another or with the CNTs. The product is then cooled. In such a process, the reactants may form additional holes or pores in the CNTs, increasing the specific surface area of the sintered body. Alternatively, the reactants may deposit materials on the surface of the CNTs without affecting the underlying CNT structure.

In another embodiment, the CNT mixture is initially heated and sintered in a nonreactive environment (e.g., in a vacuum, in the presence of helium, or in the presence of argon). Subsequent to sintering, the heat and pressure are changed to suitable reaction conditions and reactants are added to the environment. Such reactants may include a variety of metals (as liquid or vapor), metal carbonyls, silanes, or hydrocarbons. The reaction of the reactants with one another or with the carbon of the CNT may fill some or all of the interstices of the CNT lattice with products of the reactions. Such processing with additional reactants may in some cases be conducted during sintering, but may also be performed separately. The heat and pressure are maintained until the desired level of reaction (both cross-linking within the CNTs, and the reaction between the CNTs and the reactant) has occurred. The reacted mixture is then cooled and removed from the reaction environment for further processing, storage, packaging, shipment, sale, etc.

FIG. 11 schematically depicts a mass 300 of covalently bound CNTs 302. The CNTs 302 bind through sintering with other CNTs 302 (multi-wall or single-wall CNTs) through mutual contact points 304, binding the aggregate together into a highly cross-linked structure. The resultant binding may create a material of significant strength, toughness, impact resistance, and electrical and thermal conductivity.

During the sintering process, the green body may shrink, corresponding with a decrease in the size of voids among the CNTs. However, the sintered body may remain porous due to the porosity of each CNT (i.e., the center of the CNT) and due to voids between and among CNTs. The sintered body may have pores or voids having a median minimum dimension of less than about 1 μm, less than about 500 nm, less than about 100 nm, less than about 50 nm, or even less than about 10 nm. That is, each void may have two or more dimensions (e.g., a length, a width, and a height, each perpendicular to the others, or a diameter and a length), measured in different directions. The voids need not be regularly shaped. The “minimum dimension” is defined as the minimum of the two or more dimensions of a single void. The “median minimum dimension” is defined as the median of these minimum dimensions for a group of voids.

A sintered body as described herein may have a high specific surface area, due to voids between CNTs and within CNTs (i.e., because the CNTs are hollow). For example, a sintered body may have a specific surface area of at least about 100 m²/g, at least about 500 m²/g, at least about 750 m²/g, at least about 900 m²/g, or even at least about 1000 m²/g. The specific surface area can be controlled by the characteristic diameters or mixture of diameters of the CNTs used in forming the solid carbon product. For example, small-diameter single-wall CNTs have specific surface areas up to approximately 3000 m²/g, while large-diameter multi-wall CNTs have specific surface areas of approximately 100 m²/g.

A sintered body may have a high electrical conductivity. For example, a sintered body may have an electrical conductivity of at least about 1×10⁵ S/m (Siemens per meter), at least about 1×10⁶ S/m, at least about 1×10⁷ S/m, or even at least about 1×10⁸ S/m. The electrical conductivity can be controlled by the types of CNTs used, the chirality of the CNTs used, the sintering conditions, and the quantity of resulting covalent bonds in the solid carbon product. For example, single-wall CNTs with a metallic chirality have a much higher electrical conductivity than multi-wall CNTs. As a further example, an increase in the number of covalent bonds appears to correlate with an increase in conductivity.

A sintered body may also have a high thermal conductivity. For example, a sintered body may have a thermal conductivity of at least about 400 W/m·K (watts per meter per Kelvin), at least about 1000 W/m·K, at least about 2000 W/m·K, or even at least about 4000 W/m·K. The thermal conductivity of the resulting solid carbon product may be controlled by the types of CNTs used and the chirality of CNTs used. For example, single-wall CNTs with a metallic chirality have much high thermal conductivity than large multi-wall CNTs.

CNTs may alternatively be pressed after the sintering process by, for example, extrusion or molding, as described above with respect to FIGS. 5 through 9. In some embodiments, the sintering process may be part of the formation of the desired object. For example, a section of the extrusion barrel may heat the CNTs to the sintering temperature in an inert atmosphere for an appropriate amount of time to cause sintering. Such heating may be, for example, induction heating or plasma arc heating. Thus, sintered CNTs may be extruded. The sintered CNTs may optionally be mixed with another material such as a metal, a ceramic, or glass. The material may be pressed or pulled through a die under either extreme heat or cold. The material, forced into a given shape, is held in place for a period of time and at sintering temperatures and pressures, and then returned to normal atmospheric conditions. The products may be continuous, such as wires, or may be discrete pieces, such as bolts, propellers, gears, etc. Molding of sintered or sintering CNTs typically involves either using the CNT material in concentrated form (i.e., with minimal impurities) or in forming a moldable composite with another material, such as a metal. The moldable material is placed or poured into a rigid mold, held at a particular temperature and pressure, and then cooled back to normal atmospheric conditions.

In some embodiments, an incremental manufacturing method may be employed wherein CNTs (either compressed or not) are placed in a nonreactive environment, such as in an inert gas autoclave. The CNTs are sintered to form covalent bonds between the CNTs in the surface layer and the underlying layer. For example, a laser may irradiate a portion of the CNTs in a pattern. Additional CNTs are deposited over the sintered CNTs, and in turn sintered. The sintering process is repeated as many times as necessary to achieve a selected thickness of sintered CNTs. The sintered CNTs are then cooled to a temperature below which the CNTs do not react with oxygen or other atmospheric gases. The sintered CNTs may then be removed from the nonreactive environment without contaminating the sintered CNTs. In some embodiments, the sintered CNTs are cooled and removed from the nonreactive environment before deposition of each additional portion of CNTs.

In certain embodiments, sintered solid carbon products are formed in a belt-casting operation. A layer of CNTs is placed on a moveable belt. The belt moves the CNTs into a chamber containing a nonreactive environment. The CNTs are sintered in the chamber, then cooled (e.g., in a portion of the chamber), and removed from the chamber. The process may be operated continuously, such as to form a sheet of sintered CNTs.

In some embodiments, solid carbon products are further treated by electrodeposition to fill interstices in the solid carbon products with another material. A solution having materials to be deposited is prepared. The solvent of the solution may be water, an organic solvent, or an inorganic solvent. The solute may include a material such as a metal salt, an organic salt, a metalorganic salt, etc. Electroplating solutions are known in the art and not described in detail herein. The solid carbon product to be treated is contacted with the solution, such as by immersing the body in the solution. An electric potential (a direct-current voltage or an alternating-current voltage) is applied to the body to induce electrodeposition of one or more components of the solution. The composition, potential, temperature, and/or pressure are maintained until a selected amount of the material is deposited onto the solid carbon product. The solid carbon product is then removed from the solution and rinsed to remove excess solution.

Solid carbon products formed as described herein each include a plurality of cross-linked CNTs. The CNTs define a plurality of voids, which may have a median minimum dimension of less than about 1 μm, less than about 500 nm, less than about 100 nm, less than about 50 nm, or even less than about 10 nm. Some or all of the CNTs may include a metal, such as a metal particle from which the CNTs were formed, or a metal coating on the CNTs. The solid carbon products may be structural members (e.g., beams), fasteners (e.g., screws), moving parts (e.g., propellers, crankshafts, etc.), electrically conductive members (e.g., electrodes, wires, etc.), or any other form. The solid carbon product may include another material dispersed in a continuous matrix surrounding and in contact with the CNTs. The solid carbon products may have improved strength, toughness, impact resistance, and electrical and thermal conductivity in comparison to conventional materials.

In some embodiments, the solid carbon products also include other morphologies of carbon, interspersed with or otherwise secured to the CNTs. For example, buckyballs may be connected to some of the CNTs. As another example, one or more graphene sheets may be formed over all or a portion of a solid carbon product.

Both the compressed solid carbon products and the sintered solid carbon products described herein have a wide variety of potentially useful applications. For example, the compressed solid carbon products may be used as filters, molecular sieves, catalysts, and electrodes in applications where the additional mechanical integrity achieved through sintering is not necessary. The sintered solid carbon products can be used in the applications in which compressed solid carbon products can be used and in a wide variety of additional applications requiring additional mechanical integrity, electrical properties, and other material-property enhancements achieved through sintering.

Sintered solid carbon products may be useful components of armor because of their mechanical integrity, ability to absorb compressive loads with a high spring constant, and ability to dissipate heat. That is, sintered solid carbon products may be used to form projectile-resistant materials, such as armor plates, bullet-proof vests, etc. The light weight of the solid carbon products could improve mission payloads, increase vehicle range, and alter the center of gravity. For example, armor materials including sintered solid carbon products may be beneficial in preventing injury and death of occupants of vehicles such as Mine Resistant Ambush Protected vehicles (“MRAPs”), which are prone to tipping. Sintered solid carbon products as described herein may be effective in light-weight armament systems such as mortar tubes, gun barrels, cannon barrels, and other components. Sintered solid carbon products may also be beneficial in aerial vehicles, such as aircraft, spacecraft, missiles, etc.

EXAMPLES Example 1: Sintering of Compacted CNTs

CNTs were formed as described in U.S. Patent Publication No. 2012/0034150 A1. Samples of approximately 1.0 grams to 1.25 grams of CNTs each were pressed in 15-mm diameter dies using a 100-ton (890-kN) press. The pressed samples were placed in an inert gas furnace (Model 1000-3060-FP20, available from Thermal Technology, LLC, of Santa Rosa, Calif.) and heated under vacuum at a rate of 25° C. until the samples reached 400° C. This temperature was maintained for 30 minutes to allow the samples to outgas any oxygen, water, or other materials present. The furnace was then filled with inert gas (argon or helium) at 3-5 psi (21 to 34 kPa) above atmospheric pressure. The furnace was heated at a rate of 20° C./min until the sample reached 1500° C. This temperature was maintained for 30 minutes. Heating continued at 5° C./min to a sintering temperature, which was maintained for a dwell time of 60 minutes. The samples were then cooled at 50° C./min to 1000° C., after which the furnace was shut down until the samples reached ambient temperature. The sample masses, compaction pressures, and sintering temperatures for the samples are shown in Table 1 below. The inert gas was helium for the samples sintered at 2400° C. and was argon for the other samples.

TABLE 1 Samples prepared in Example 1 Compaction Sintering Sample Mass (g) Pressure (MPa) Temperature (° C.) 1 1.076 500 1800 2 1.225 750 1800 3 1.176 250 1800 4 1.113 500 2100 5 1.107 750 2100 6 1.147 250 2100 7 1.103 500 2400 8 1.198 750 2400 9 1.121 250 2400 10 1.128 250 1900 11 1.209 500 1900 12 1.212 750 1900 13 1.101 250 2000 14 1.091 500 2000 15 1.225 750 2000 16 1.078 250 1700 17 1.179 500 1700 18 1.157 750 1700

Samples 1 through 18 were harder and more robust than were the samples before the heating process. At the highest sintering temperature of 2400° C. (samples 7 through 9), the sintered pellets are flakier than the other sintered samples. All the samples prepared in Example 1 were qualitatively observed to be hard.

Pycnometry tests show that the skeletal density decreases from 2.2 g/cm³ for raw powders and raw compactions to 2.1 g/cm³, 2.08 g/cm³, and 2.05 g/cm³ for the samples sintered at 1800° C., 2100° C., and 2400° C., respectively. Bulk density also decreased after sintering, in almost every case to less than 1.0 g/cm³. Pellet thickness increased 5% to 9% during sintering, with the higher pressure compactions expanding more than the lower pressure compactions. The bulk densities of Samples 1 through 9 are shown in Table 2 and in FIG. 12.

TABLE 2 Properties of samples prepared in Example 1: After Compaction Compac- After Sintering tion Skeletal Bulk Sintering Skeletal Bulk Pressure Density Density Temperature Density Density Sample (MPa) (g/cc) (g/cc) (° C.) (g/cc) (g/cc) 1 600 2.1992 1.043 1800 2.1095 0.960 2 900 2.2090 1.095 1800 2.0993 0.994 3 300 0.990 1800 2.1131 0.921 4 600 1.063 2100 2.0680 0.971 5 900 1.084 2100 2.0817 0.992 6 300 0.999 2100 2.0829 0.910 7 300 0.985 2400 2.0553 0.932 8 600 1.069 2400 2.0479 1.009 9 900 1.102 2400 2.0666 0.991

Example 2: Spark Plasma Sintering of CNTs

CNTs were formed as described in U.S. Patent Publication No. 2012/0034150 A1. Graphite foil (available from Mineral Seal Corp., of Tucson, Ariz.) was lined into 20-mm diameter dies, and 2.0 g to 4.0 g of CNTs were placed over the foil. The samples were placed in a spark plasma sintering (SPS) system (model SPS 25-10, available from Thermal Technology, LLC, of Santa Rosa, Calif.). An axial pressure of approximately 5 MPa was applied to the CNT samples, and the SPS system was then evacuated to less than 3 mTorr (0.4 Pa). The sample was heated at 150° C./min to 650° C., and this temperature was maintained for one minute to allow the vacuum pump to re-evacuate any materials out-gassed. The pressure was increased to the compaction pressure of 30 MPa or 57 MPa, while simultaneously increasing the temperature at a rate of 50° C./min to 1500° C. The temperature and pressure were maintained for one minute. The temperature was then increased at 50° C./min to the sintering temperature, and maintained for 10 min or 20 min. After the dwell, the pressure was reduced to 5 MPa, and the sample allowed to cool at 150° C./min to 1000° C., after which the furnace was shut off until the samples reached ambient temperature.

The sample masses, compaction pressures, compaction rates, sintering temperatures, and dwell times for the samples are shown in Table 2 below.

TABLE 3 Samples prepared in Example 2: Compaction Sintering Dwell Mass Pressure Compaction Temperature time Sample (g) (MPa) rate (MPa/min) (° C.) (min) 19 2.449 57 13.0 1800 10 20 3.027 57 13.0 2100 10 21 4.180 57 13.0 1800 20 22 4.210 30 6.0 1800 10 23 4.417 30 6.0 1800 20

The SPS-sintered pellets formed in Example 2 were about 10 mm thick and had bulk densities between 1.3 g/cm³ and 1.5 g/cm³. To illustrate the strength of these samples, sample #20 was planned to be sintered 2100° C., but at about 1900° C., the die broke. The ram traveled significantly, crushing the graphite die. After the test was completed, the die was broken away from the sample. The sample remained visibly intact, though slightly thinner than expected. This would indicate that the sintering occurs at temperatures less than 1900° C., that the strength of SPS-sintered pellets is high, even at extreme temperatures, and that the sintered samples are strong enough to resist an applied force without fracturing.

The bulk densities of the samples with the graphite foil still attached were determined. For the samples weighing about 4 g (i.e., samples #21, #22, and #23), bulk densities were between 1.35 g/cm³ and 1.50 g/cm³. The volume resistivity and electrical conductivity of the samples were also measured. These data are shown in Table 4. The samples are more conductive than amorphous carbon, and nearly as conductive as graphite.

TABLE 4 Properties of samples prepared in Example 2: Density Resistivity Electrical Sample (g/cm³) Resistance (Ω) (Ω · m) Conductivity (S/m) 19 1.588 2.42 × 10⁻³ 4.98 × 10⁻⁵ 2.01 × 10⁻⁴ 20 1.715 2.02 × 10⁻³ 4.77 × 10⁻⁵ 2.10 × 10⁻⁴ 21 1.494 3.24 × 10⁻³ 1.23 × 10⁻⁴ 8.14 × 10⁻³ 22 1.350 3.80 × 10⁻³ 1.62 × 10⁻⁴ 6.19 × 10⁻³ 23 1.429  3.7 × 10⁻³ 1.57 × 10⁻⁴ 6.37 × 10⁻³

Although the foregoing description contains many specifics, these are not to be construed as limiting the scope of the present invention, but merely as providing certain embodiments. Similarly, other embodiments of the invention may be devised which do not depart from the scope of the present invention. For example, features described herein with reference to one embodiment also may be provided in others of the embodiments described herein. The scope of the invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions, and modifications to the invention, as disclosed herein, which fall within the meaning and scope of the claims, are encompassed by the present invention. 

1. A sintered solid carbon product comprising a metal and a plurality of carbon nanotubes, wherein at least some of the plurality of carbon nanotubes are covalently bonded to other carbon nanotubes of the plurality.
 2. The sintered solid carbon product of claim 1, wherein the carbon nanotubes define a plurality of voids therebetween having a median minimum dimension of less than about 100 nm.
 3. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product has a bulk density of greater than approximately 1.3 g/cm³.
 4. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product comprises a projectile-resistant material.
 5. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product comprises an electrically conductive material.
 6. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product has an electrical conductivity of at least approximately 1×10⁵ S/m.
 7. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product has an electrical conductivity of at least approximately 1×10⁷ S/m.
 8. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product comprises an electrode.
 9. The sintered solid carbon product of claim 1, wherein the metal comprises a catalyst residual selected from the group consisting of the elements of Groups 5 through 10 of the periodic table.
 10. The sintered solid carbon product of claim 1, wherein at least some of the carbon nanotubes contain a metal within growth tips of the carbon nanotubes.
 11. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product has a bulk density of less than approximately 2.2 g/cm³.
 12. The sintered solid carbon product of claim 1, further comprising at least one material selected from the group consisting of a ceramic and a lubricant, wherein the at least one material is interspersed in a continuous matrix surrounding and in contact with the plurality of carbon nanotubes.
 13. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product comprises a structural member.
 14. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product has a thermal conductivity of at least approximately 400 W/m·K.
 15. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product has a thermal conductivity of at least approximately 2000 W/m·K.
 16. The sintered solid carbon product of claim 1, wherein the sintered solid carbon product has a thermal conductivity of at least approximately 4000 W/m·K.
 17. A sintered solid carbon product comprising a metal and a plurality of carbon nanostructures, wherein the carbon nanostructures comprise carbon atoms bonded into a hexagonal lattice, wherein at least some of the plurality of carbon nanostructures are covalently bonded to other carbon nanostructures of the plurality.
 18. The sintered solid carbon products of claim 17, wherein at least some of the plurality of carbon nanostructures comprise multi-wall carbon nanotubes.
 19. The sintered solid carbon product of claim 17, wherein at least some of the plurality of carbon nanostructures comprise nanobuds attached to carbon nanotubes.
 20. The sintered solid carbon products of claim 17, wherein at least some of the plurality of carbon nanostructures comprise single-wall carbon nanotubes. 